Electrical coils are often used as inductors and transformers in electrical circuit design. In integrated circuits, these coils are typically planar, existing in two dimensions only. These planar coils have several problems. The inductance per unit length is low and the signal loss is high. The parasitic capacitance, including inter-electrode and line-ground capacitance, is high causing low resonant frequency. In addition, planar coils of this type occupy a large amount of space on an integrated circuit. The low quality factor, evidenced by the low inductance per unit length and high signal loss, makes these planar coils a poor choice for on-chip filter design. Because of these problems, it is difficult, if not impossible, to make good on-chip filters using planar coils as inductors.
Planar coils also suffer from inefficient use of magnetic fields which are part of any electrical coil. In a planar coil, the magnetic fields of adjacent turns of the coil are in opposite directions. Therefore, there is no mutual inductance. There is only the self-inductance of each turn of the coil. The lack of mutual inductance manifests itself in low inductance per unit length. The high inter-electrode capacitance is caused by the fact that every adjacent turn in the planar coil is coupled by the substrate. In addition, the magnetic fields in a planar coil can be terminated by the ground beneath the integrated circuit substrate, thus causing induced currents which greatly increase loss of electrical signal.
Due to the aforementioned problems, planar coils are an inefficient choice for on-chip transformers and inductors in integrated circuit design.